**1. Introduction**

The carbide end mills are the most popular and versatile tools for processing a wide range of metals, alloys, and non-metallic materials [1–5]. The outstanding capabilities of modern carbide mills are provided by a variety of original design solutions, the correct selection of tool geometry, and the use of a wide range of modern wear-resistant coatings [6–8]. The application of wear-resistant coatings with a thickness of 3–7 μm to the working surfaces of end mills gives them the characteristics necessary for specific processing conditions and to ensure that the coating effectively complements the physical and mechanical properties of the hard alloy substrate and together, they will have increased wear resistance during cutting [9–11]. Among the numerous coatings, thin-film diamond-like coatings (DLC) represent a separate group of particular interest. Due to their excellent anti-friction properties and good resistance to abrasive wear, these coatings are now successfully used in mechanical engineering and metalworking as surface protection of machine parts operating under conditions of increased friction with mating parts and cutting tools made of high-speed steels and hard alloys, processing non-ferrous metals and alloys, composite materials, and others [12–16].

For the formation of DLCs of various structures and properties, research organizations and manufacturing enterprises are currently well equipped and have technological equipment based on the principles of physical vapor deposition (PVD), chemical vapor deposition (CVD), as well as their combination [17–22]. However, whatever method of production we use, the structure of the formed DLC assumes the simultaneous presence of various forms of carbon in them, which can exist in different hybridizations. Modern technologies for the deposition of DLCs provide the formation of films with a different ratio between *sp*<sup>3</sup> (diamond) and *sp*<sup>2</sup> (graphite-like) hybridizations combined in an amorphous structure (Figure 1). The properties of the formed coating depend on the type of bonds that hold the carbon atoms.

**Figure 1.** Different hybridizations of carbon in diamond-like coatings (DLC) films.

The higher the proportion of *sp*<sup>3</sup> bonds, the closer the properties of such material is to diamond—increase in its hardness and resistance to abrasive wear. In turn, the higher the content of *sp*<sup>2</sup> bonds, the closer the properties of the material are to graphite—its hardness and wear resistance are reduced at temperatures that occur during cutting. When creating DLCs, the main task of technologists is to get a coating with a structure in which the number of *sp*<sup>3</sup> links is increased compared to *sp*<sup>2</sup> links. Furthermore, of great importance for the characteristics of coatings is the amount of hydrogen and other components that are always present in coatings deposited by industrial technologies [23–27].

DLCs have specific features: with an increase in film thickness, the level of residual stresses increases noticeably, the strength of its adhesive bond with the substrate decreases, and, as a result, the coating does not effectively resist the loads acting on it during the cutting of materials (especially cyclic loads during interrupted cutting, for example, milling).

According to the known physical laws, a film of thickness *h* will peel off when the elastic energy per volume unit due to stress σ exceeds the energy required for the formation of two new surfaces by exfoliation [27,28]:

$$h < \frac{4\chi E}{\sigma^2} \tag{1}$$

where γ is surface energy (J/m2), σ is stress (Pa), *h* is the coating thickness (μm), and *E* is the modulus of elasticity (Pa).

This dependence sets the upper limit of the film thickness, above which it peels off spontaneously. When a tool with a coating is exposed to power loads, the stresses from the external load are additionally superimposed on the residual stresses in the coating, and accordingly, the destruction occurs at significantly lower loads.

Undoubtedly, the stress state of a DLC is influenced by the essential physical and mechanical characteristics—hardness (*H*), modulus of elasticity (*E*), as well as their ratio (*H*/*E*), called "index of plasticity". There is one more feature that must be taken into account when developing coatings for a hard alloy tool—due to an abrupt change in the modulus of elasticity of the coating layer and the surface layer of the tool material, significant tangential shear stresses arise at their interface, which can lead to peeling of the coating during cutting. Therefore, to reduce technological stresses at the interface and ensure increased strength of the adhesive bond between the coating and the tool, it is necessary to provide, as close as possible, the characteristics of the modulus of elasticity of these materials [29–34].

To minimize the problems described above, various technological approaches are used when creating DLCs: they are doped with different elements and compounds (silicon, tungsten, and others), and can form adhesive sublayers (intermediate coatings) [35–40]. A universal technical solution cannot be found. When developing the architecture of a DLC and the technology of its deposition, it is necessary to be guided by the specific operating conditions of the coated product, and it is necessary to evaluate the effectiveness of the chosen approach under the conditions of the action of real operating loads or as close as possible to them.

In this work, the goal was to comprehensively study the effect of the formation of an adhesive sublayer based on (CrAlSi)N on carbide end mills before applying the outer DLC film using the PECVD method in the presence of a multicomponent gas mixture containing tetramethylsilane, in comparison with applying only a single-layer DLC, and determining the degree of influence of the adhesive sublayer on the critical physical, mechanical, and structural characteristics of the DLC. Furthermore, we quantified the influence of single-layer (without sublayer) and double-layer (with sublayer) DLCs on the wear rate of end mills during operation and the surface roughness of machined parts made of aluminum alloy AlCuMg2 and low-carbon steel 41Cr4. For evaluating the effectiveness of the DLC under different heat and power loads, we specifically selected two fundamentally different processed materials for research. Compared to other structural materials, aluminum alloys are well amenable to machining, relatively low power and temperature loads on the cutting tool arise. At the same time, they have specific features—repeated adhesion of aluminum particles to the cutting edge of the tool during the cutting process, followed by their tearing out from the working surfaces of the tool, which intensifies tool wear and reduces the quality of the processed surface [41–44]. When machining low-alloy structural steels by end mills, the thermal load is noticeably higher, and the main problem is the wear on the flank of the tool as a result of abrasion caused by hard components in the work material. Furthermore, adhesion wear of the working surfaces of the end mill is observed due to the local grasp of the processed and tool materials, followed by separation of the smallest particles from the tool, which are carried away by the descending chips [45,46].

A separate comment is required to avoid doubt about the validity of the authors' choice of the DLC for the tool processing ferrous steel. According to classical concepts, at elevated temperatures, carbon is intensely dissolved in iron, and graphitization of diamond crystals occurs. However, in this case, the authors do not use a diamond tool but study the behavior of a multicomponent DLC film containing various modifications of carbon, and compounds based on silicon, which is a component of the gas mixture during condensation of the DLC film. We purposefully selected iron-containing steel as the material to be processed to study the functioning of the adhesive sublayer and the outer DLC layer under the loads typical for milling this material. We should add that the authors have research experience in using DLCs for turning hardened bearing steels, and in their previous works, a specific effect was observed from the use of coatings for ceramic tools [21,47–49]. This makes it possible to use a carbide tool as an object of research in this work.

The choice of (CrAlSi)N as an adhesive sublayer material for the functioning of the DLC is not accidental and is explained by the great potential of using this compound for the needs of tool production [49–51]. This nitride coating compares favorably with traditional nitride films such as (TiAl)N, (TiCr)N, and (TiNbAl)N since it is a nanocomposite, its deposition does not entirely mix the components but forms two phases. Its structure consists of AlCrN nanocrystals embedded in an amorphous SiN matrix, which provides a high level of strength of interatomic bonds between atoms of the amorphous and crystalline phases [52,53]. The introduction of silicon into the coating reduces the internal residual stress at the "hard alloy-coating" interface, and the formation of oxides by the coating components at high temperatures slows down the tribochemical reactions on the contact surfaces of the cutting tool. These features, and previously obtained experimental data [49,54,55], allow us to count on the effectiveness of the composite compound (CrAlSi)N for carbide end mills as an adhesive sublayer before applying the external DLC.
